1. Field of the Invention
This invention relates to rotary valves, and more particularly to butterfly valves with annular seats especially for use under a wide range of pressures and for ambient and low temperatures. This invention is an improvement of the invention disclosed in my prior U.S. Pat. No. 3,997,142, issued Dec. 14, 1976.
2. Description of the Prior Art
Butterfly and other types of rotary valves with annular yieldable seats constructed of Teflon, Kel-F, and other polymeric materials, are well-known and commonly used for controlling the flow of various fluids in a wide variety of industries. Although some of these valves are satisfactory whem employed at ambient or moderate temperatures and modest pressures, experience has shown that seats of this type employed in butterfly valves often leaked when exposed to cryogenic temperatures, for these seats, like most other solid material, contract when they are cooled. This volume change affects all of the dimensions of the seat, and since the extent of contraction is also dependent upon the material from which the seat is constructed, it is quite difficult to provide a butterfly valve seat that will maintain a fluid-tight seal over a wide range of temperatures, and especially when subjected to significantly elevated pressures.
Although seats of yieldable polymeric materials are considered to be the most suitable for use in butterfly valves for wide temperature ranges, when the valve is exposed to extreme cold, as encountered in cryogenic applications, there is a tendency for the seat to shrink away from the metallic valve elements between which it is secured, thereby establishing a leakage path. Another complication is presented if the butterfly valve's disc is opened while the valve is being cooled down, for in this situation the seat tends to contract and warp out of its circular form. Maintaining the open valve at cryogenic temperatures causes the seal element to stiffen in its warped form thereby making it very difficult, if not impossible, for the seat to return to its circular shape when the valve is closed, and thus the leakage problem is compounded.
Earlier attempts to overcome these problems include the use of mechanical springs, fulcrum systems, etc., with or without temperature-responsive members, to press the seat against the valve disc and thereby hopefully to eliminate leakage. The use of springs at extremely low temperatures is undesirable because they tend to lose some of their physical characteristics. For example, when metal springs are exposed to cryogenic temperatures they become stiff and brittle; they could break and thereby permanently disable the valve. Furthermore, many of the seat energizing devices heretofore known as unduly complicated, and relatively expensive to manufacture.
What is desired is a temperature-responsive rotary valve having a valve seat that maintains a fluid-tight contact with the valve body and with the flow control element over a wide range of temperatures from ambient all the way down to cryogenic. It is desirable that the valve seat also maintains these fluid-tight contacts throughout a wide range of pressures. In order to do this, it is necessary to increase the degree of contact between the valve seat and the flow control element as the pressure of the fluid increases. It is also desirable to provide a temperature-responsive valve that is relatively simple and inexpensive to build.